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I’ve put in a few days time now studying carbon sequesterization in limestone. This morning I’ve run several calculations based on raw data dug up in Al Gore’s house – a.k.a. the internet. Joking aside, I flatly disagree with the IPCC two hundred year shelf life of CO2. It makes zero sense to me, and of course basic numbers don’t agree. My hypothesis is that limestone production is a strong feedback mechanism to CO2 production. It’s nearly a self proving result because of the very low concentration of plant food in the atmosphere for so many millions of years, but most don’t seem to consider that as evidence.

This paper is an estimate of limestone sedimentation rates across the ocean based on biological origin, other papers provide similar numbers. An estimate of 10 grams per meter squared per year gives a result of 3.3 GT of sequestered carbon per year across the whole ocean. This 10g rate of deposition is at the lowest end of the scale for what I’ve found. Since our industry oututs about 7gt of CO2 per year that amounts to about half of the total human output.

If we consider that today at 380ppm we have an estimate of .1 mm/year and at 120,000 ppm we have 360mm/year we can estimate the feedback to CO2 from ocean limestone creation.

380ppm / 0.1 mm/year = 3800 CO2 concentration vs sequesterization

120,000ppm /360 mm/yr = 333 CO2 concetration vs sequesterization.

So at much higher CO2 concetrations the oceans were on average 9 times more efficient at carbon capture than they are today.

A linear plot looks like this:

I’m not silly enough to believe a simple linear projection can solve this problem but IF we consider that these numbers have basis in measured rock formations, we are forced to realize this is evidence of strong natural feedback to CO2 concentration in the atmosphere. To put it further into perspective, if these historic numbers are to be believed, and we linearly interpolate between them, a doubling of CO2 from 380 to 760 would result in an increase in deposition rate to 1.06 mm/year or ten times today’s rate. I doubt very much that this is an accurate number but it shows the magnitude of the feedback from measured data vs CO2 concentrations.

Were strong feedback the case, why wouldn’t we have seen a stabilization of CO2 levels or a reduction in the buildup? Time for some handwaiving on my part.

– atmospheric CO2 and ocean Co2 buildup are separate issues. The ocean concentration is what affects ocean plants so there could be a long lag time before the feedbacks become fully active.

– The deposition may have already increased but the increase is masked by atmospheric output of global industry.

– Variance in ocean concentrations may drive some atmospheric concentrations. Ice cores blend CO2 over a fifty to a hundred years before the CO2 is encapsulated, perhaps some of what we are seeing in rise is due to release from oceans.

– Historic deposition numbers are overestimated for unknown reasons.

What we cannot discount though is that there was substantially greater deposition rates by biological processes at 15% CO2 concentration as compared to .038%. This huge deposition happened in what (according to today’s climate science) had to be higher acidity oceans, which means life somehow adapted to the CO2 and managed to create shells and limestone even in high carbon environments.

Of course, I don’t know much more than this but find the whole concept of natural CO2 capture both interesting and poorly understood in climate science. This could very well explain why the ‘sinks’ are not saturating as has been predicted by many in climatology.

31 Responses to “Feedback”

jeff idsaid

Nullius in Verbasaid

Limestone drives the feedback on a geological timescale, but is too slow to affect things on a human timescale. For what it’s worth, the currently extremely low levels of CO2 are believed to be the result of the uplift of the Himalayas, which increases weathering and erosion, effectively performing the reverse reaction to the deposition mechanism.

I’d suggest you look up the “solubility pump” for a more realistic mechanism. (The biological pump, also.) The residence time of CO2 in the atmosphere is actually about 5 years, because it is constantly being released in vast amounts from hot tropical waters and then absorbed again in the cold polar waters. CO2 solubility in water is strongly dependent on temperature. An excess of CO2 can last a lot longer, because the absorption rate at the poles does not necessarily increase as fast as needed, but it is by no means as simple as “CO2 is emitted into the atmosphere and stays there”.

The solubility pump is of course the mechanism for Al Gore’s Big Graph: in which rising temperature leads, 800 years later, to higher CO2.

800 years sounds like a long time to wait for it all to go away again, but I suspect that you can get smaller responses much faster.

Laws of Naturesaid

thank you for antother interesting topic!
As I said in the other thread, there might have been a viral infection of the phytoplankton in the last years, perhaps it counteracts the increased deposition rate from the higher CO2-cocentration.
I guess you are aware of this site!?http://www.seafriends.org.nz/issues/global/acid2.htm#carbon_situation

Nullius in Verbasaid

I’m not saying it’s wrong (although I am a little sceptical), but it’s only half of the equilibrium balance. You also have to measure the sillicate/carbonate erosion worldwide to assess how carbonates affect the atmosphere.

The polar oceans absorb 90 GtC/yr, compared to mankind’s puny 6 GtC/yr contribution. If I wasn’t including the balancing emission by the tropical oceans, that would look like a pretty good carbon sink!

There are also deeper sources of carbon. Diamonds, of course, are carbon from the Earth’s mantle.

CO2 is trapped as a fluid under high pressures in the Earth’s upper mantle and is sometimes released explosively. CO2 from the mantle contains a tracer isotope: Radiogenic Xe-129 from the decay of extinct I-129 [“The xenon record of extinct radioactivities in the Earth,” Science 174, 1334-1336 (1971); http://www.sciencemag.org/cgi/content/abstract/174/4016/1334 ]

You need to remember that the only significant carbon deposition in the oceans via carbonate sedimentation occurs at depth. How this occurs is a way lot more complex and debatable – so you can work out from that how much we can rely on IPCC and the AGW lobby, than you have assumed here.

The key issue is the variation of the depth at which sinking organic carbon is converted back to CO2 – known as the remineralization depth. Without going into this technically ad infinitum as it is a fairly complex subject, and boring everyone, a very good place for anyone genuinely interested to start would be:

Brian Hsaid

#2;
“the currently extremely low levels” says it all. There is a CO2 famine, and life (including human) would be far better served with levels about 5X current. But human input is unable to affect levels one way or the other, unfortunately.

Steve Fitzpatricksaid

Yup, the remineralization depth is very important, and not even known about by most.

#2
Yes, ocean circulation driven pumping of CO2 is one of the dominant sinks (if not THE dominant sink) for CO2 on relatively short time scales. The 800 year turnover is conservative… most research says considerably longer. But in any case, fossil fuels will be a thing of the distant past long before the ocean approaches equilibrium with the CO2 released from fossil fuels.

Jeff,
No need to invoke carbonate deposits to account for the current rate of sequestration; that rate is consistent with removal at a rate of >2 PPM per year by the oceans, at the current atmospheric concentration of ~390 PPM (in the absence of continued fossil fuel combustion). If all fossil fuel combustion were to stop, the atmospheric CO2 concentration would fall to about 350 PPM within 25 years, and to about 325 PPM within 50 years. The projections of very slow CO2 uptake are not consistent with currently calculated uptakes… just one more scare story, generously couched in words like “could” and “might” and “possibly”.

Excellent article, Jeff. This is something that should be very easy to collect emperical data to support your theory. Experiments have been done showing plant growth in a greenhouse with increased levels of CO2. Similarly, it should be possible to have 2 aquariums with shellfish and/or coral with different levels of dissolved CO2, and measure the shell/coral growth.

Coincidentally, there is a posting on WUWT today by Professor Bob Carlson and at the end it mentions that he is a stratigrapher and marine geologist. It would be great to see a paper published on historical and current rate of deposition calcium carbonate.

Steve Fitzpatricksaid

Comparison of estimated emissions from all sources (well over 4PPM per year potential increase) versus actual atmospheric increase (~2 PPM per year), proves that most of the CO2 is being sequestered. The hard part is knowing exactly where… significant quantities could be sequestered in increased biomass (in addition to what goes into the ocean), depending on how much CO2 is released due to land use changes versus sequestered due to increased plant growth from higher CO2 and slightly warmer temperatures. The most widely accepted view is that the release from land use change has been greater than the increase in plant growth, but this is by no means certain. It does appear that the ocean dominates the net sequestration. This article and associated references: http://wattsupwiththat.files.wordpress.com/2009/11/knorr2009_co2_sequestration.pdf is a good place to start.

#12, I used linear interpolation between the 1mm per day and the rate today. We have two points 120,000 ppm 360mm/yr and 380 ppm .2mm/yr. Working the math to follow the line causes a huge jump in deposition. It’s an interesting topic, and if there are any lurking marine geology/biology guys, I’d like to hear from them.

Seansaid

An interesting topic that might easily be solved by simple solubility equations. The solubility product for a substance like calcium carbonate (limestone) is defined by a solubility product. What it says is that if you have more of one of the ions, in this case carbonate, in solution, then it is more likely to drop out of solution even if the calcium ratio is lower. Another odd thing about calcium carbonate, its actually more soluble in cold water than it is in hot water. Perhaps that’s why coral reefs form in warm seas.
For the life of me, when anyone says you can permanently sequester carbon, I always tell them to look out at the view of the Grand Canyon. I believe that close to 1/3 to 1/2 of the exposed rock is limestone that mother nature has sequestered for 300 to 700 million years. This limestone layer extends north at least a thousand miles and south into Mexico. Then there are the white cliff of Dover in the UK, Italian marble, chalk deposits, etc. In fact I thik its difficult to go anywhere on the earth where you might not encounter carbonate deposits. Mother nature is really really good at sequestering CO2 (without a government subsidy I might add).

Seansaid

I need to correct a typo or two and expand on the complicated chemistry of calcuim and carbonate ions. First of all meant to say that the solubility of a substance (like CaCO3) is a function of the solubility product. A bit more digging also revealed a bit more complex story. While calcium vs. carbonate ions are defined by a very low solubility of in water (47 mg/l) calcium bicarbonate is much more soluble although not stable enough to be isolated. So when you have a lot of CO2 dissolved in water you can also have a lot of calcium ions because of the bicarbonate. However, if the water gets warm enough (thermal vents near boiling) the bi-carbonate in the presence of calcium wants to break down and form calcium carbonate solid and release CO2 back into the gaseous state. I wonder if this occurs with a little help from biological processes where a couple carbon dioxide molecules that dissolves in the cold water near the poles may be tranported to the warmer lattitudes by ocean currents in the form of soluble bicarbonates and then they break down CO2 (g) and calcium carbonate (s) when the hit the very warm waters near the equator.

Charlie Asaid

It would be interesting to see if there are any other datapoints of CO2 levels in the atmosphere vs. the rate of limestone deposition.

Obviously, plotting actual observations of estimated prehistory CO2 and associated deposition rates would be the best way to estimate, but for a very wide spread as 0.1mm/yr @ 380ppm to 360mm/yr @ 12,000 ppm I’d guess a logarithmic relationship rather than the linear approach you took. That works out to an increase of about 2.7 in the deposition rate for every doubling of CO2.

So I calculate a deposition rate about 0.27mm/yr for a doubling of CO2 rather than the 1mm/yr you estimate.

Amabosaid

DeWitt Paynesaid

snopes.com can be biased when politics are involved. The argument in the posted link is mainly semantic based on the difference in meaning between ‘create’ (which is what Gore actually said) and ‘invent’. Founder would be much closer in meaning to creator than inventor so it’s not clear that your link actually rebuts the point.

Amabosaid

Al Gore did not even claim to have ‘created’ the internet, much less ‘invented’ it. (“During my service in the United States Congress, I took the initiative in creating the Internet. ” is as close as he gets.)

Personally, I think that senators taking an interest in developing technologies can have a positive effect on said technologies, and that it is therefore likely that Al Gore has contributed positively to the development of the internet and related technologies.

Philip Mulhollandsaid

This is a complex subject, and I haven’t too many sources to help substantiate the following statements, but here’s a bit of help from a geoscience perspective.

1. The minerals Calcium Carbonate, found in limestone and Magnesium Carbonate, found in dolomite, are chemical salts.
2. Salts are formed by the following chemical process:
Acid plus Base equals Salt plus Water only.
For example H2CO3 + Ca(OH)2 = CaCO3 + 2H2O
3. The carbonate fraction of limestone is derived from atmospheric Carbon Dioxide gas while the Calcium fraction is derived from the chemical weathering of the feldspar minerals found in volcanic Basalt rock.
4. The production of limestone rock is the mechanism by which the Earth sequesters oxidised carbon, removing this gas from our atmosphere and ensuring that Earth can never become like Venus. (Note that the temperature at which limestone minerals thermally dissociate into Carbon Dioxide and Calcium Oxide is more than twice that of the surface temperature of Venus, so limestone is a safe repository for carbon dioxide gas on the earth).
5. Unlike most salts the solubility of Calcium Carbonate in water decreases as the temperature of the water rises, this aids the inorganic precipitation of the mineral.
6. Half of the limestone rock in the sedimentary basins of the world has been produced by inorganic precipitation.
7. The total mass of carbonates present in the sedimentary rocks has increased throughout geological time as the Earth has aged.
8. Inorganic precipitation of carbonate sediments occurs today in carbonate beach sands at the littoral edge of Carbonate Ramp Environments.
9. The grains of inorganic carbonate sand form in the swash zone of the beach. They are ovoid in shape and typically grow onion layered around an aragonite seed crystal that is usually of organic (bioclastic) origin.
10. These inorganic ovoid carbonate sands form the oolitic limestones that are found throughout the geological record.
11. The waters of the World Ocean are brines stratified by density with those waters having the greatest density occurring at the greatest depth.
12. There are two distinct mechanisms for generating dense marine brines, one mechanism, found in ocean waters at polar latitudes, generates cold dense sea water. The second mechanism, found in shallow seas at tropical latitudes, generates warm dense sea water. The warm tropical brines are often associated with carbonate sedimentation and they typically have a higher fluid density than the cold polar brines.

During the Cretaceous Period the world was warmed by the presence of vast carbonate ramps in mid latitudes on the margins of the Tethys Ocean. The warm dense brines generated in these shallow seas ensured that the ocean bottom waters throughout the world were kept at a temperature of 16 degrees Celcius. Consequently the Cretaceous atmosphere contained more carbon dioxide gas than is found in the atmosphere our modern cold ocean world.

With the loss of the Tethys Ocean and the destruction of the carbonate ramps during the Tertiary Period, the cold brine mechanism came to dominate the production of dense brines throughout the World Ocean. This cooling our modern world allowed the oceans to dissolve more carbon dioxide out of the atmosphere, to the detriment of all land plants, especially during the most recent ice ages.

The level of carbon dioxide gas in the Earth’s atmosphere is a thermometer of the world ocean temperature and not a planetary atmospheric thermostat.

Re: Philip Mulholland (Aug 2 22:42),
Philip, you’ve made some good points, but an aspect of 2) is worth emphasising, because it’s often overlooked. A base is required, and there is no natural readily available source of suitable base. Instead, it is produced only gradually by weathering of basalts. This ensures that sequestration as CaCO3 can only happen slowly, and the rate is largely independent of CO2 pressure.

Pascvakssaid

If we start pumping regular, old, everyday air (the kind we all breathe)down to a depth of 1km (let’s start in the Gulf of Mexico) the impact on sea life would be significant and the increased carbon sequesterization would more than offset the effects of pollution from NOLA. It would also eliminate the “Dead Zone” in the Gulf caused by the fertilizer rich Mississippi runoff. If 1km is a little much, let’s build ten times as many pumps and just go down 100m; this would also allow for a significant reduction in Co2. Come on! We can borrow the money from China. Right?

Philip Mulhollandsaid

Thanks for your comment. You ask for a story and because Nick Stokes commented on aspect #2 of my post above, I will develop that theme further and then tell you a story.

I first want to discuss the source of the calcium and magnesium cations that allow for the continuing sequestration of carbon dioxide gas into the Earth’s ultimate mineral carbonate sink, limestone and dolomite rock.

On Earth there are clear physical and chemical differences in the types of rocks that form the crust of our planet. Late nineteenth century geologists recognised that these differences can be used to divide the Earth’s crust into two groups, which they termed Sima & Sial. Sima corresponds to the dense basic igneous rocks of the oceanic crust, rocks dominated by silica (Si) minerals rich in magnesium (Ma), such as olivine in basalt. Sial refers to the less dense acidic igneous and metamorphic rocks of the continental crust, rocks dominated by silica (Si) minerals rich in aluminium (Al), such as alkali feldspar in granite.

Nineteenth century geologists also recognised in the folded and uplifted rocks of ancient mountain ranges, such as the Appalachians, the existence of former sedimentary basins which they called “Geosynclines”. Sedimentary geosynclines were considered by the Victorians to be the same width as the subsequent fold mountains, because they deemed the spatial arrangement of the Earth’s continents and oceans to be fixed. The Victorians were unable to identify any modern geosynclines and so did not deduce that the geosynclines they were describing were the remnants of wide and laterally extensive ancient oceans. Geologists now believe that fold-mountain belts contain metamorphosed rocks, deposited originally as sediments in former marine basins. These ancient oceans have been narrowed and crushed by tectonic forces and the sediments welded onto the adjacent landmass in a process of continental accretion and growth.

That the granitic continents of the Earth have increased in bulk throughout the life of our planet, as each successive phase of mountain building has added more mass to them, shows that the earth has a mechanism “the geochemical factory” that converts the basic rocks of the Sima into more acidic Sial rock, allowing the continents to grow.

The geochemical factory has a set of separate, but interlocking, processes of crystal and mineral refinement. These processes consist of:
1. Crystal Fractionation, which occurs within the body of the Earth, in molten magma chambers where hot igneous rocks are formed and dense basic minerals are separated within the melt, by selective temperature controlled crystallisation and gravity settling to the base of the magma chamber of the heaviest crystals.
2. In the body of the earth mineral crystals are stable at the pressure and temperature of their formation. When rocks are transported by tectonic processes to the low pressure, low temperature regime near the earth’s surface, then these crystals become unstable and slowly change into new mineral forms.
3. Chemical Weathering, which occurs at the Earth’s surface, where the crystals of exposed mineral rocks are weathered and chemically altered by temperature, water and gases (such as oxygen & carbon dioxide) to form mineral soils.
4. Sediment Transport, involving water, ice and wind, which removes the weathered soils and dissolved salts from elevated continental surfaces and return them to the seas and ocean basins. Here the separated clastic minerals are deposited in stratal layers as new sedimentary rocks and the soluble salts remain dissolved in the ocean waters.
5. Diagenesis, a process of re-mineralisation, whereby buried granular stratal sediments are lithified into solid durable rock. The rocks are cemented by precipitation of low soluble dissolved minerals such as silica and calcite, from the surrounding pore waters under conditions of raised temperature and pressure due to increased sediment load.
6. Highly soluble mineral salts are also removed from the Earth’s ocean waters by a separate process that occurs in shallow ephemeral seas. Here the evaporation of water creates concentrated brines that crystallise to form evaporite rocks such as gypsum (calcium sulphate), halite (sodium chloride) and under conditions of extreme desiccation sylvite (potassium chloride).

In essence the Earth’s geochemical factory processes basaltic ocean crust, refines the basaltic rocks, first into volcanic andesites & rhyolites, via crystal fractionation. Then, via the surface weathering processes that form sedimentary sands and clays and followed by the metamorphic mountain building process, these sands and clays are ultimately converted into new granitic continental crust. In doing so, and as an integral part of the weathering and refinement process, the geochemical factory exports soluble basic sodium (Na+) calcium (Ca++) and magnesium (Mg++) cations into the ocean waters of our planet.

Consider now the specific case of Calcium and how this alkali metal, the critical base of limestone is derived from a sequence of flood basalt rocks erupted over a continental surface in a tropical location, such as the Deccan Traps in India. Basalt rock typically consists of the minerals Olivine ({Mg,Fe}2SiO4); Plagioclase Feldspars (CaAl2Si2O8) and Pyroxene (XY{Si,Al}2O6, where X can be Calcium, Sodium, Iron II or Magnesium & Y can be Aluminium or Iron III, amongst others).

Under the temperature and rainfall conditions of a monsoonal climate, the basaltic minerals weather to produce soils rich in Haematite (Iron III oxide, Fe2O3), Goethite (Iron III oxyhydroxide, FeOOH), Kaolinite (a sheet silicate, Al2Si2O5{OH}4) & Gibbsite (Aluminium hydroxide, Al{OH}3) minerals. This weathering process releases calcium and magnesium cations into the soil structure where they combine with soil water anions such as sulphate and bicarbonate. The release of cations by chemical weathering of minerals in basic rocks is the source of the calcium that forms new calcite and gypsum minerals, and is the process by which carbon dioxide and sulphur dioxide gases are sequestered into the sedimentary rocks of the Earth.

Now for the Story:-
65 million years ago, the northern branch of the Atlantic Ocean, that now separates Greenland from Scandinavia, and is occupied at its mid-point by the gigantic super volcano we call Iceland, did not exist. Instead, in place of the modern Atlantic there was a system of continental rift valleys that were the initial stages in the development of the new ocean. Unlike the yet to be created Atlantic, the mantle plume that now sources the Icelandic volcanism did exist 65 million years ago. This volcanic hot spot produced basaltic lavas that flooded out over the fractured continental crust of north-west Europe, forming extensive lava plateaux, such as the Antrim basalts in northern Ireland.

At that time the climate of northern Europe was paratropical, not because of the volcanism, but because the Tethys Ocean, with its mid-latitude solar energy collecting shallow continental shelves, ensured that the early Tertiary (Paleocene-Eocene) world was warm. The existence of this warm climate can be deduced from both geological and biological (paleontological) evidence. In Antrim the surface soils of the basalt plateau were weathered by a monsoonal climate to form laterite soils that contained bauxite. The bauxite mineral deposits of Antrim are direct proof of that the Paleocene warm world climate permitted chemical weathering of basalt at latitude 54 degrees North, something that is not possible in our modern cold world. In addition to the presence of bauxite, the rocks of Antrim also contain deposits of Tertiary brown coals that contain the organic remains of Taxodium trees.

In the Everglades of modern Florida grows a conifer tree not known in Europe and unlike most other gymnosperms, it is deciduous. The Americans call this tree the Bald Cypress, it is a member of the Taxodium genus and its ancient antecedents grew in Antrim in the Early Tertiary. The Bald Cypress is so called because in winter it is leafless and bald, and in spring its newly growing shoots appear as cypress fronds before they unfurl into the extended feather shape of the mature summer leaf. Gymnosperms are an ancient family of woody plants divided into many genera. They have existed since the Jurassic and typically specialise in living in harsh mountain environments. With their robust structure they can survive the lowest winter temperatures as evergreens. So why are some gymnosperms, such as the Bald Cypress deciduous?

In the far north of Canada, on the shores of the Arctic Ocean, there exist organic peats derived from the remains of early Tertiary forests. Even 55 million years ago this land and its forests were located at a northern latitude well inside the Arctic circle. The peat deposits show that these forests and others surrounding the ancient Boreal Ocean contained Taxodium, Metasequoia, Larix, Glyptostrobus and Ginkgo flora. All the modern species of these ancient gymnosperm genera are deciduous, but only the Larix species still grow in the taiga forests of the far north.

/Quote/
Polar regions were covered with extensive forests during the Cretaceous and early Tertiary, and supported trees comparable in size and productivity to those of present-day temperate forests. With a winter of total or near darkness and a summer of continuous, low-angle illumination, these temperate, high-latitude forests were characterized by a light regime without a contemporary counterpart. Taxodium, Larix and Metasequoia, three genera of deciduous conifers that occurred in paleoarctic wet forests, have extant, closely related descendents. Forest trees, growing at latitudes as high as 80° N, would have been subjected to about 4 months of continuous daylight and a similar period of continuous winter darkness (Pielou 1994).
/End Quote/

This lack of winter light over an extended number of generations is this most plausible explanation for the development of the deciduous habit in unrelated gymnosperm genera that grew together in high latitude forests during the early Tertiary. The ancestors of the Bald Cypress trees of Florida, developed their deciduous habit to enable them to survive the dark winters of the high arctic in the now vanished warm world of 55 million years ago.

Philip Mulhollandsaid

Thanks for your comment. You ask for a story and because Nick Stokes commented on aspect #2 of my post, I will develop that theme further and then tell you a story.

I first want to discuss the source of the calcium and magnesium cations that allow for the continuing sequestration of carbon dioxide gas into the Earth’s ultimate mineral carbonate sink, limestone and dolomite rock.

On Earth there are clear physical and chemical differences in the types of rocks that form the crust of our planet. Late nineteenth century geologists recognised that these differences can be used to divide the Earth’s crust into two groups, which they termed Sima & Sial. Sima corresponds to the dense basic igneous rocks of the oceanic crust, rocks dominated by silica (Si) minerals rich in magnesium (Ma), such as olivine in basalt. Sial refers to the less dense acidic igneous and metamorphic rocks of the continental crust, rocks dominated by silica (Si) minerals rich in aluminium (Al), such as alkali feldspar in granite.

Nineteenth century geologists also recognised in the folded and uplifted rocks of ancient mountain ranges, such as the Appalachians, the existence of former sedimentary basins which they called “Geosynclines”. Sedimentary geosynclines were considered by the Victorians to be the same width as the subsequent fold mountains, because they deemed the spatial arrangement of the Earth’s continents and oceans to be fixed. The Victorians were unable to identify any modern geosynclines and so did not deduce that the geosynclines they were describing were the remnants of wide and laterally extensive ancient oceans. Geologists now believe that fold-mountain belts contain metamorphosed rocks, deposited originally as sediments in former marine basins. These ancient oceans have been narrowed and crushed by tectonic forces and the sediments welded onto the adjacent landmass in a process of continental accretion and growth.

That the granitic continents of the Earth have increased in bulk throughout the life of our planet, as each successive phase of mountain building has added more mass to them, shows that the earth has a mechanism “the geochemical factory” that converts the basic rocks of the Sima into more acidic Sial rock, allowing the continents to grow.

The geochemical factory has a set of separate, but interlocking, processes of crystal and mineral refinement. These processes consist of:
1. Crystal Fractionation, which occurs within the body of the Earth, in molten magma chambers where hot igneous rocks are formed and dense basic minerals are separated within the melt, by selective temperature controlled crystallisation and gravity settling to the base of the magma chamber of the heaviest crystals.
2. In the body of the earth mineral crystals are stable at the pressure and temperature of their formation. When rocks are transported by tectonic processes to the low pressure, low temperature regime near the earth’s surface, then these crystals become unstable and slowly change into new mineral forms.
3. Chemical Weathering, which occurs at the Earth’s surface, where the crystals of exposed mineral rocks are weathered and chemically altered by temperature, water and gases (such as oxygen & carbon dioxide) to form mineral soils.
4. Sediment Transport, involving water, ice and wind, which removes the weathered soils and dissolved salts from elevated continental surfaces and return them to the seas and ocean basins. Here the separated clastic minerals are deposited in stratal layers as new sedimentary rocks and the soluble salts remain dissolved in the ocean waters.
5. Diagenesis, a process of re-mineralisation, whereby buried granular stratal sediments are lithified into solid durable rock. The rocks are cemented by precipitation of low soluble dissolved minerals such as silica and calcite, from the surrounding pore waters under conditions of raised temperature and pressure due to increased sediment load.
6. Highly soluble mineral salts are also removed from the Earth’s ocean waters by a separate process that occurs in shallow ephemeral seas. Here the evaporation of water creates concentrated brines that crystallise to form evaporite rocks such as gypsum (calcium sulphate), halite (sodium chloride) and under conditions of extreme desiccation sylvite (potassium chloride).

In essence the Earth’s geochemical factory processes basaltic ocean crust, refines the basaltic rocks, first into volcanic andesites & rhyolites, via crystal fractionation. Then, via the surface weathering processes that form sedimentary sands and clays and followed by the metamorphic mountain building process, these sands and clays are ultimately converted into new granitic continental crust. In doing so, and as an integral part of the weathering and refinement process, the geochemical factory exports soluble basic sodium (Na+) calcium (Ca++) and magnesium (Mg++) cations into the ocean waters of our planet.

Consider now the specific case of Calcium and how this alkali metal, the critical base of limestone is derived from a sequence of flood basalt rocks erupted over a continental surface in a tropical location, such as the Deccan Traps in India. Basalt rock typically consists of the minerals Olivine ({Mg,Fe}2SiO4); Plagioclase Feldspars (CaAl2Si2O8) and Pyroxene (XY{Si,Al}2O6, where X can be Calcium, Sodium, Iron II or Magnesium & Y can be Aluminium or Iron III, amongst others).

Under the temperature and rainfall conditions of a monsoonal climate, the basaltic minerals weather to produce soils rich in Haematite (Iron III oxide, Fe2O3), Goethite (Iron III oxyhydroxide, FeOOH), Kaolinite (a sheet silicate, Al2Si2O5{OH}4) & Gibbsite (Aluminium hydroxide, Al{OH}3) minerals. This weathering process releases calcium and magnesium cations into the soil structure where they combine with soil water anions such as sulphate and bicarbonate. The release of cations by chemical weathering of minerals in basic rocks is the source of the calcium that forms new calcite and gypsum minerals, and is the process by which carbon dioxide and sulphur dioxide gases are sequestered into the sedimentary rocks of the Earth.

Philip Mulhollandsaid

It is a mid-June day in 1991, West Caicos, a small uninhabited tropical island in the Turks and Caicos archipelago, bakes in the hot summer sun. I am on a field trip to the British West Indies organised by Dr Hal Wanless of the University of Miami, to study the modern geology and depositional environments of a natural carbonate factory. A visit that, even now, I consider to have been the best field study trip of my entire geoscience career. Located in the trade wind belt, the Turks and Caicos Islands lie at the south-eastern end of the Bahaman chain of Atlantic Ocean carbonate-platform islands. With the Tropic of Cancer passing to the north of the group, at midday the June sun is directly overhead and your shadow is as small as it possibly can be. By evening, the summer thunderstorms arrive tracking west across the ocean, passing by on their way to the Caribbean.

For most of the year, the climate of West Caicos is dominated by dry trade winds. These are derived from the down-welling of the Hadley Cell, centred over the Atlantic Ocean to the north-east. The low rainfall and high evaporation rate make the climate too dry for sugar cane production, a failed economic enterprise tried by past entrepreneurs at this remote island location. Salt production, the original economic activity of the Turks and Caicos, was attempted at West Caicos, but that enterprise failed also. The salt pans were located on the site of a major wash-over fan in the northwest of the island. The regolith here consists of permeable limestone rubble and not the impermeable coastal micritic mudflats, the natural location of choice for salt production on the other islands in the group. This site, with its poor hydrogeology, probably accounts for the failure of the West Caicos salt pan enterprise.

Now West Caicos is a nature reserve and the native bromeliad flora are left to grow undisturbed. We are here to undertake a west to east traverse across the island to see how the individual elements of its geology have been created by the natural marine processes of active carbonate deposition occurring over the past few thousand years, since the sea level rise at the end of the last ice-age flooded the Caicos platform.

We begin our journey in the sea, swimming with mask and flippers off the island’s west coast; we are here to observe the corals thriving in the shallow warm waters of the reef flat, everyone’s ideal coral island setting. Swimming here is easy in the shallows, except for the slight ocean swell, as we make our way out to the drop-off, and spot the barracuda fish below, patrolling the reef edge, marking its location. Then everything suddenly changes, the seabed disappears from sight as the water depth precipitately increases towards the 300 metre level, the water colour becomes a deep blue and the water temperature drops as we move into the ocean current. Here I experience a sudden cramp in my legs and I am grateful for the life jacket I’m wearing and the presence of my safety buddy, as swimming becomes difficult in the colder water. So why is the water now so cold and where has all the warm water gone? Leaving these questions unanswered we swim to the safety of the Z boat and head back to shore.

Our next stop is just off the beach, here the corals are no longer thriving, they are being buried by carbonate beach sand and the burrows of innumerable marine creatures pockmark the seabed. This change to carbonate sand is not evidence of a dying environment, this is a thriving pristine environment, it is simply no longer the coral’s home and a new force of nature, sediment derived from the inorganic carbonate beach factory, dominates the scene. Carbonate geologists estimate that approximately 50% of all the carbonate rock on Earth is generated by inorganic means and our next stop is the factory floor, the sand generating swash zone of the carbonate beach environment.

We arrive on the western beach of West Caicos, standing in the swash zone where the sea water reaches its warmest temperature. We observe the continuous back and forth motion of the sea as each wave arrives, rolling the grains of carbonate sand and creating a smooth beach with a distinctive sedimentary pattern or facies. Hal draws our attention to the beach rock in a small cliff adjacent to our landing point. Here we can see, preserved in the vertical rock face and deposited at a time of previously higher sea level, the sedimentary facies of the same near shore environments we have just observed offshore. In the base of the cliff we find the fossil corals, above them surrounding and smothering them we see the lithified carbonate sand grains and the distinctive cone shaped burrows of the long dead marine animals. Above this zone are the smooth layers of sand from the old swash zone forming a structured Z shaped pattern in the cliff face marking the exact tidal limit of the ancient beach. This is a classic geological example of the “Principle of Superposition and Original Horizontality”, where the younger sediments of the proximal shallow-water beach environment extend over the older distal deeper-water coral reef, as the sea bed shallows and the island grows seaward. The effects of this principle are regularly observed in marine carbonate deposits, with each upward episodic sea level change defining the datum for a repeated pattern of sequential stratal growth.

We climb off the beach, up onto the rock outcrop and on its upper level we find gigantic boulders of beach rock with the same three strata as below, but tumbled out of their original setting. Hal observes that these boulders have been ripped out from the cliff and deposited up here by a storm surge from a former hurricane. My personal view is that this could be a tsunami deposit, given that we are due north of Hispaniola and at the western end of the active Puerto Rico submarine trench, this explanation of a powerful wave, generated by a submarine earthquake, also seems plausible. It is my belief that in geoscience it is always a good idea to entertain more than one explanation for any set of field observations.

We are now standing at the top of the cliff, above the sea, on the oldest part of the island. Turning to face east, we see the land fall away in a gentle slope and in the distance, on the horizon, a line of sand dunes rises behind a blue lake. Following a straight track, laid out by the former sugar plantation enterprise, we are soon down hill, back at sea level, walking out on a causeway across the brine lake. Half way across there is a break in the track, the site of a former culvert, where the lake water flows though the causeway gap from north to south. Hal explains that on every visit to West Caicos he has always observed the same continuous direction of flow, so a tidal explanation for the movement of the water can be discounted. The lake occupies the site of an old bay on the island’s former east coast, now separated from the lagoon by lines of barrier dunes, but its waters are still hydrologically connected to the sea by an underground phreatic limestone aquifer. The wind driven marine current flowing west towards West Caicos island across the shallow Caicos lagoon creates a hydraulic gradient on the islands east coast that forces sea water underground, through the island’s limestone core to emerge in and flow through this central blue lake, before the water makes its way back underground to regain the open sea at the island’s west coast.

Beyond the lake the track rises to a cut through heavily vegetated small hills, the maturity of the bromeliad flora demonstrates the significant age of these now inactive dunes. At the crest line, a new vista appears, in the distance a second line of modern sand dunes lies beyond a sabkha mudflat. We descend and cross the sabkha, its fragile algal crust breaking under the pressure of our footsteps, to reveal soft gypsum mud below. The presence of natural gypsum (hydrated calcium sulphate) in this ocean island setting is a surprise and is a testimony to the effectiveness of the high evaporation rate of the West Caicos climate in concentrating the seawater brine.

Leaving the sabkha we climb the line of modern dunes, the loose sand and the sparse vegetation of grasses demonstrate the young age of this second barrier to be crossed before we reach the modern east coast of West Caicos. Beyond the crest, a rapid descent brings us down to a wide wind swept beach. A continuous drying wind, blowing in our face, moves the loose sand off the shore, adding to the dunes behind us and raising the island’s surface above sea level by means of aeolian sedimentation.

Here on the wide eastern beach, sitting below a small Casuarina tree and facing the shallow lagoon, we see the true extent of the carbonate sediment factory, a prolific producer of inorganic carbonate sand. Oolitic (egg shaped) grains roll in the beach swash zone growing layer on layer to produce an onion ringed sand grain wrapped around an original seed crystal of aragonite. Out beyond the beach the shallow warm sea, with water depths of less than 10 metres, extends eastward for 100 km, it is dotted with small patch reefs of coral rising clear of the sandy bottom. Parrot fish, with their strong beaks, bio-erode the coral and excrete crystals of indigestible aragonite, mineral seeds that form an endless supply of crystals around which new oolitic sand grains grow, in a symbiotic union of organic and inorganic sedimentation.

It is now over 19 years since that summer day, yet the memories of my short visit to West Caicos remain vivid. Looking back now, at the end of my geoscience career, it is time to place all the elements of that day into an environmental synthesis and answer the question of what happened to the warm water when I swam beyond the reef edge and experienced that dangerous cramp in the cold waters of the Atlantic Ocean.

Marine inorganic carbonate sediment creation is a warm-water, shallow-sea process. There are two major types of carbonate environment present in the seas of Earth; namely carbonate platforms and carbonate ramps. Carbonate platforms are found throughout the tropical oceans, while carbonate ramps are found on continental shelves in tropical epiric seas.

The Caicos Islands are an example of a modern active carbonate platform that forms an area of shallow sea surrounded by the deep waters of the Atlantic Ocean. The dimensions of the platform are large, in the south it extends from West Caicos to Seal Cays, a distance of about 100 km, while in the north it extends from Providenciales to East Caicos a distance of about 80 km. The platform covers an area of approximately 5,400 sq km, of which only 430 sq km is land and about 5,000 sq km is covered by shallow sea. This shallow lagoon is a gigantic solar energy collector, each day the tropical sun warms the sea water and all day and night the dry north-east trade wind enhances the surface evaporation, increasing the sea-water salinity and driving the current westward across the lagoon towards West Caicos and the open ocean beyond.

As the temperature and salinity of the ocean water increases in the lagoon a process of evaporitic precipitation of salts from marine waters becomes possible. This deposition of salts occurs in a distinct pattern of increasing solubility. Calcium carbonate, the least soluble salt, precipitates first. The water soluble calcium bicarbonate is converted to calcium carbonate precipitate with the release of gaseous carbon dioxide to the atmosphere. This process takes place in the warmth of the beach’s swash zone and accounts for the prolific carbonate sand sedimentation found here and throughout the Bahamas. The next salt that precipitates from the seawater concentrate is gypsum (hydrated calcium sulphate). This process takes place on the West Caicos sabkha, behind the dunes, where the ponded seawater, driven onto the island by the wind and tide, concentrates by further evaporation. The third salt to precipitate is halite (sodium chloride) this is the most soluble mineral of the three and therefore the most difficult to precipitate. The waters of the brine lake demonstrate that there is the potential for this process to occur on West Caicos, and would do so if a suitable natural salt pan existed here.

Halite deposition does occur on other islands in the group that have more favourable environmental conditions, such as the islands of Providenciales, North, Middle & East Caicos on the northern rim of the Caicos platform. These islands are all growing by natural tide assisted sediment accretion of carbonate mud. They have mangrove swamps on their southern coasts and are slowly extending out across the Caicos lagoon. Behind the zone of mangroves, the drying wind and hot sun permit the deposition of gypsum and halite in flat natural salt pans.

As a consequence of the process of evaporation the sun warmed sea waters leaving the Caicos lagoon, on its western margin, are denser than the cold ocean waters that have flowed around the carbonate platform. At the reef edge this water density difference causes the warm lagoon brine to sink towards the depths below the colder less saline ocean water and accounts for the sudden thermal contrast I experienced in the sea off West Caicos island that June day in 1991.